Drilling fluids, often referred to as drilling muds in the oil industry, are used in well drilling operations. The drilling fluid, which may be a water-, oil-, or synthetic-based formulation, circulates within the well bore, carries cuttings to the surface, lubricates the drilling equipment, and acts as a cooling agent. So-called “lost circulation” occurs when drilling fluids or muds enter into a porous or fractured formation, and are lost to the drilling operation rather than returning to the surface for recycling and reuse. Lost circulation is a significant industry problem. It is estimated that lost circulation costs the industry about $800 million per year in the United States alone, and that the lost circulation products could represent as much as an additional $200 million annually. Lost circulation can lead to failures in testing wells, and can also lead to decreased productivity.
A variety of “lost circulation materials” have been employed to try to reduce levels of lost circulation. These “lost circulation materials” have included such things as coarse cellulosic fibers, fine cellulosic fibers, coarse nut shells, synthetic graphite, cellulose derivatives, mineral fibers, fine calcium carbonate (e.g., 1 mm in diameter), medium-sized calcium carbonate (e.g., 2 mm in diameter), and coarse calcium carbonate (e.g., 3 mm in diameter). Lost circulation materials often include different particle types and sizes to address different sizes of fracture. If part of the material is rigid but compressible or expandable under pressure, the material can perhaps mold itself into the fractures to help seal leaks. Cellulosic fibers alone will often lack the needed rigidity. To compensate for the lower rigidity, a greater concentration of the material may be required to effectively inhibit lost circulation. Due to the low density of cellulose, cellulosic fibers are often pelletized when used as lost circulation materials to reduce transportation, storage, and handling costs. Cellulosic materials can be subject to decay, mold, and insect (e.g., termites) degradation during storage. Inelastic materials such as waste plastics and rubber used in the lost circulation materials typically do not have the swelling/expansion properties that are needed to better plug cracks and fissures. Resilient graphitic carbon (RGC) of various sizes has been used in lost circulation compositions due to its resilience, but RGC can be expensive.
U.S. Patent Application publication no. 2006/0096759 discloses a lost circulation composition with a first portion of particles having a weight mean particle size of less than about 20 microns, a second portion of particles having a weight mean particle size in the range from about 25 microns to about 400 microns, and a third portion of particles having a weight mean particle size in the range of from about 450 microns to about 1,500 microns.
U.S. Pat. No. 4,422,948 discloses a lost circulation material comprising a mixture of flexible flakes, fibers, and granular particles made of shredded paper, mineral wool, and calcium carbonate.
U.S. Pat. No. 7,229,492 discloses a well cement composition comprising a hydraulic cement, water, and inelastic lost circulation material particles made of granulated waste materials such as polyethylene, polystyrene, or polypropylene.
U.S. Pat. No. 3,375,888 discloses a three-component lost circulation material comprising nutshells, shredded carpet, and lacquered regenerated cellulose.
U.S. Pat. No. 3,574,099 discloses the use of nutshells and asbestos fibers as lost circulation material.
U.S. Pat. No. 4,526,240 discloses the use a lost circulation material containing a hydrophilic absorbent composite comprising a fibrous absorbent and a water-swellable hydrophilic polymer absorbent. A compacted, fibrous material such as layered tissue or paper laminate, is mechanically shredded to form a loose, soft mass of fibers (“fluff”), which is combined with a swellable polymer for lost circulation control.
U.S. Pat. No. 3,629,102 discloses a three-component mixture of coarse nutshells, fine nutshells, and sugar cane fibers as a lost circulation material.
U.S. Pat. No. 4,579,668 describes a two-component lost circulation material derived from discarded wet-cell battery casings. The first component is a thermoplastic polymer in a flexible, elongated form, and the second component is a granular thermoset plastic with a specific gravity in the range 1.2-1.4.
U.S. Pat. No. 5,826,669 discloses the use of resilient graphitic materials for fluid loss and lubrication purposes.
U.S. Patent Application publication no. 2008/0113879 discloses the use of plastic granules (e.g., polypropylene) as lost circulation additives in drilling fluid.
U.S. Patent Application No. 2009/0054269 A1 discloses the use of what were called “composite particles” containing a cellulosic fiber and a filler as a lost circulation material. Examples of cellulosic fibers included fibers from trees and plants, sugar beet, sugarcane, citrus pulp, potatoes, grains, peanut hulls, corn cobs, tobacco stems, apple pumice, natural grasses, cotton, and peat. Examples of fillers included kaolin clay, calcium carbonate, barites, titanium dioxide, and ground thermoset polymer laminates. For example, in a “composite particulate” comprising a cellulosic fiber and a filler, the cellulosic fiber could be paper fiber, and the filler could be a ground laminate containing paper or fabric and melamine resin.
There is an unfilled need for improved, effective, and economical compositions and methods for reducing lost circulation in wells.